1. Field of the Invention
The present invention is directed to a token passing scheme for a local area network having a known number of stations and requiring a deterministic response time and, more particularly, to a system in which the failure of a node or the reactivation of a failed node will not effect system response time or invalidate data on the network.
2. Description of the Related Art
Token passing schemes are used in local area networks where multiple computers or other devices communicate over a common communication media. In order to determine which station or node has the right to transmit, local area networks generally have a token passing scheme whereby a token, granting the right to transmit, is passed from one station to another. Prior art token passing schemes, as typified by the IEEE standard for local area networks entitled "Token-Passing Bus Access Method and Physical Layer Specifications" approved as ANSI/IEEE Standard 802.4-1985, are designed for systems which on start up or restart do not know how many stations exist on the network and do not know the addresses for the existing stations. In such systems, during a start up, a number of collisions between data from different stations occur because some or all of the stations start communicating during the same time period. To straighten out the collisions, collision detection and avoidance hardware is required. In standard token passing systems, the station possessing the token, transmits as much data as desired and then transmits a special frame which transfers the token to the next station. This special token frame wastes time. When a node in the standard system is reactivated after a failure, it starts communicating immediately, causing a collision and requiring that the network restart to determine the new configuration. A restart requires that all stations broadcast their addresses to other stations. The activation of a failed station thus interrupts communications for a considerable period of time and could cause critical time dependent data to be delayed. The standard token passing scheme is not suitable for critical process control situations, such as in a nuclear power plant, where data integrity and timeliness must be maintained.